University of Cincinnati

UNIVERSITY OF CINCINNATI Date:___________________ I, _________________________________________________________, hereby submit this work as part of the requirements for the degree of: in: It is entitled: This work and its defense approved by: Chair: _______________________________ _______________________________ _______________________________ _______________________________ _______________________________ EFFECT OF AGING ON ABRASIVE WEAR RESISTANCE OF SILICON CARBIDE PARTICULATE REINFORCED ALUMINUM MATRIX COMPOSITE A thesis submitted to the Division of Graduate Studies and Research of the University of Cincinnati In partial fulfillment of the requirement for the degree of MASTER OF SCIENCE in the Department of Chemical and Materials Engineering of the College of Engineering 2007 by Varun Sethi B.Tech, National Institute of Technology, Jamshedpur, 2005 Committee Chair: Dr. R Y. Lin Abstract The effect of aging on the wear resistance of SiC particle reinforced aluminum composites was investigated. The as cast Al/SiCp composite used in this study was purchased from Duralcan with A-356 matrix and 23Vol% of SiC reinforcement. This composite was solutionized at 565ºC and then aged at 180 ºC for different time intervals and changes in hardness and wear resistance were measured using a Rockwell B hardness tester and Pin-on-disc wear tester respectively. For reference purpose an alloy of Al-10wt% silicon was used. Wear resistance of the aged composites was found superior to the as cast composite with the peak aged composite showing the maximum wear resistance and overaging resulted in a decrease in wear resistance. Results also showed that the wear resistance of the composites was greater than the monolithic alloy at all loads and wear rate was found to increase with pressure. The wear resistance of Al-Si(10wt%) alloy was found to increase with aging, but no variation in wear rate was found among the aged alloys. Scanning electron micrographs of worn surfaces of composites revealed that the principal mechanism of matrix removal was microcutting and microcracking. Very few SiC reinforcements were found on the worn surfaces suggesting that that the penetration depth of the abrasive was greater than the reinforcement particle size in most cases. Finally the differences in wear resistance of the composites were rationalized on the basis of changes at the interface of SiC particle and aluminum matrix due to the presence of precipitates. Acknowledgement I would like to take this opportunity to express my sincere appreciation for my research advisor and thesis committee chair, Dr. Ray Y. Lin, for his constant support, encouragement, and able guidance throughout this study. I am grateful to my thesis committee members, Dr. Jude Iroh, and Dr. Rodney Roseman for their review and helpful criticism. I would like to thank Mr. Ratandeep Kukreja and Dr. Doug Kohls for their help in electron microscopy. I would like to appreciate CME staff Dale and Molly for their help. I am also grateful to the CME department for extending all of its resources. Table of Contents Page List of Tables i List of Figures ii List of symbols iv 1.0 Introduction 1 2.0 Background 3 2.1 Composite materials 3 2.2 Metal Matrix Composites 5 2.3 Aluminum Based Composites 6 2.4 Wear applications of aluminum based MMC’s in automotive industry 7 2.5 Fabrication of Aluminum matrix composites 11 2.5.1 Solid State Processing 11 2.5.2 Liquid State Processing 12 2.6 Interfacial Reaction 14 2.7 A356 Casting alloy 18 2.8 Aging Behavior of Metal Matrix Composites 21 2.9 Abrasive Wear 24 2.9.1 Hardness 25 2.9.2 Effect of abrasive grit dimension 26 2.9.3 Fracture toughness 27 2.10 Wear Resistance of Metal Matrix Composites 28 2.10.1 Nature of the reinforcement 30 2.10.2 Effect of reinforcement 32 2.10.3 Effect of increasing volume fraction of reinforcement on wear 34 2.10.4 Interfacial strength 36 2.10.5 Shape of dispersoid and reinforcement 37 2.10.6 Particle size 38 3.0 Experimental 42 3.1 Materials 42 3.2 Chemical analysis of composite 42 3.3 Particle size measurement 43 3.4 Density Measurement 44 3.5 Wear Test 45 3.6 Heat treatment 47 3.7 Hardness Testing 47 3.8 X-ray Diffraction 47 3.9 Microscopic Examination 48 4.0 Results 49 4.1 Composite 49 4.2 Aging Studies 51 4.3 X-ray Diffraction 58 4.4 EDS analysis of precipitate 59 4.5 Wear Behavior 60 4.6 Worn Surfaces 61 5.0 Discussion 67 5.1 Aging Studies 70 5.2 X-ray Diffraction analysis 73 5.3 Wear Studies 74 5.3.1 Effect of load on wear 74 5.3.2 Mechanism of matrix removal 75 5.3.3 Mechanism of particle removal 76 5.3.4 Dominant Mechanism 81 5.3.5 Comparison of wear behavior of composite with alloy 87 5.3.6 Comparison with previous studies 87 6.0 Conclusions 90 7.0 Future work 85 8.0 References 86 List of Tables Page Table 1 -- Selected Cast Composite Components with proven automobile applications 9 Table 2 -- Properties of A356 casting alloy 19 Table 3 -- Nominal composition of A356 aluminum alloy 42 Table 4 -- Hardness of the composites and alloys 52 Table 5 -- EDS result of precipitate present in the composite after aging 60 i List of Figures Page Figure 2.1 -- Aluminum-Silicon phase diagram 20 Figure 2.2 -- Variation of matrix microhardness as a function of aging time at 177°C 23 Figure 2.3 -- Specific wear rate in several aluminum base particulate composites sliding against steel as a function of particle volume fraction 31 Figure 2.4 -- Drawing illustrating the concept of dimensions of contact area. 39 Figure 3.1 -- Schemetic representation of pin-on-disc wear tester 46 Figure 4.1 -- Microstructure of the composite showing (a) uniform distribution of SiC particles in the matrix and (b) SiC particle embedded in aluminum matrix 49 Figure 4.2 -- (a) Optical Micrograph of the composite at 1500X (b) Optical Micrograph of the scale micrometer at 1500X 50 Figure 4.3 -- Variation of Rockwell hardness with aging time 53 Figure 4.4 -- SEM Micrographs of the composites after aging at 180ºC 54 Figure 4.5 -- SEM Micrographs of the alloys after aging at 180ºC 56 Figure 4.6 -- X-ray diffraction pattern of as cast and aged composite 58 Figure 4.7 -- SEM Micrograph of a precipitate for EDS analysis 59 Figure 4.8 -- Variation of Wear Rate with Pressure for composites 62 Figure 4.9 -- Variation of Wear Rate with Pressure taking the average of three 63 tests at a sliding distance of 80m 58 ii Figure 4.10 -- Variation of Wear Rate with Pressure for alloys 64 Figure 4.11 -- SEM micrograph of the 12 hours aged composite surface after wear 65 Figure 4.12 -- SEM micrographs of composites and alloy showing microcutting phenomenon 66 Figure 4.13 -- SEM micrographs of the composites and alloy showing Microcracking 68 Figure 5.1 -- Figure 5.1 Free Energy vs. Composition diagram for α and β phases in Al-Si System 72 Figure 5.2 -- SEM Micrograph showing the fracture of the particle 77 Figure 5.3 -- SEM micrograph of the composite showing weakening of the interfacial bond 78 Figure 5.4 -- SEM Micrograph of the composite showing a pull-out of a SiC particle from the Matrix 78 Figure 5.5 -- SEM micrographs of Aged composites showing adhesion of precipitate with SiC Particle 80 Figure 5.6 -- Schemetic showing adherence of the precipitates with the reinforcement 82 Figure 5.7 -- Schematic diagrams showing the interaction of the abrasive with the SiC particle 84 Figure 5.8 -- Schematic diagrams showing interactions between an abrasive particle and a SiC, composite at penetration depths of (a) h < d and (b) h> d. 86 iii Figure 5.9 -- The relative wear resistance vs. relative abrasive penetration depth. 86 Figure 5.10 -- SEM Micrograph showing a protruding SiC particle 88 iv List of Symbols ρ Density Vr Volume fraction of reinforcement Vm Volume fraction of matrix P Applied normal load H Bulk hardness of material Heff Effective hardness Hm Hardness of the work hardened matrix phase Hs Hardness of the second phase particles Wc Wear rate of composite Wp Wear rate of particle Wal Wear rate of aluminum matrix fal Area fraction of aluminum phase fap Area fraction of particle phase S Sliding distance d Particle size Vf Volume fraction of the particle v 1.0 Introduction Metal matrix composites are materials with metals as the base and distinct, typically ceramic phases added as reinforcements to improve the properties. The reinforcements can be in the form of fibers, whiskers and particulates. Properties of the metal matrix composites can be tailored by varying the nature of constituents and their volume fraction. They offer superior combination of properties in such a manner that today no existing monolithic material can rival. They are increasingly being used in the aerospace and automobile industries because of their improved strength, stiffness and increased wear resistance over unreinforced alloys. Aluminum is the most popular matrix for metal matrix composites because of its low density, its capability to be strengthened by precipitation, good corrosion resistance, high thermal and electrical conductivity, improved tribological properties over monolithic alloys and its high damping capacity. They are usually reinforced with SiC, Al2O3 and carbon. They find applications in automobile, defense and aerospace sectors because of their excellent combination of higher specific strength and stiffness, improved wear and seizure resistance, higher elevated temperature strength over their base alloys. Aluminum matrix composites find potential applications in automobile components like piston, cylinder liner, brake drums, crankshafts, etc[1-5].

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